Modelling [18F]LW223 PET data using simplified imaging protocols for quantification of TSPO expression in the rat heart and brain

Purpose: To provide a comprehensive assessment of the novel 18kDa translocator protein (TSPO) radiotracer, [18F]LW223, kinetics in the heart and brain when using a simplified imaging approach.
Methods: Naive adult rats and rats with surgically-induced permanent coronary artery ligation received a bolus intravenous injection of [18F]LW223 followed by 120 min PET scanning with arterial blood sampling throughout. Kinetic modelling of PET data was applied to estimated rate constants, total volume of distribution (VT) and binding potential transfer corrected (BPTC) using arterial or image-derived input function (IDIF). Quantitative bias of simplified protocols using IDIF versus arterial input function (AIF) and stability of kinetic parameters for PET imaging data of different length (40-120 minutes) were estimated.
Results: PET outcome measures estimated using IDIF significantly correlated with those derived with invasive AIF, albeit with an inherent systematic bias. Truncation of the dynamic PET scan duration to less than 100 min reduced the stability of the kinetic modelling outputs. Quantification of [18F]LW223 uptake kinetics in the brain and heart required the use of different outcome measures, with BPTC more stable in the heart and VT more stable in the brain.
Conclusion: Modelling of [18F]LW223 PET showed the use of simplified IDIF is acceptable in the rat and the minimum scan duration for quantification of TSPO expression in rats using kinetic modelling with this radiotracer is 100 minutes. Carefully assessing kinetic outcome measures when conducting a systems-level as oppose to single-organ centric analyses is crucial. This should be taken into account when assessing the emerging role of the TSPO heart-brain axis in the field of PET imaging.